In order to probe the nano-world optically, the diffraction limit has to be overcome. A round aperture can provide a nano-scale optical resolution on the order of the size of its opening; however, its low power throughput limits many applications. To improve the power throughput of nano-apertures, resonant nano-apertures, such as C-shaped apertures, have been developed. Resonant apertures can provide two to three orders of magnitude higher intensity than a round aperture, while maintaining comparable near-field spot sizes. An alternative approach for providing sub-wavelength resolution is using optical antennas, such as bow-tie antennas, which can concentrate optical energy at a sharp antenna feature. However, these approaches tend to also provide large background illumination, which also limits some applications.
A relatively recent approach is known as “tip-on-aperture” (TOA), as described by Frey et al. in an article entitled “Enhancing the resolution of scanning near-field optical microscopy by a metal tip grown on an aperture probe” (Applied Physics Letters v81n26, pp. 5030-32, 2002). In this work, a metal tip is disposed on an edge of a slit-shaped aperture, and high optical resolution combined with background free illumination is provided.
However, it remains desirable to provide high spatial resolution, low background illumination and high transmission efficiency for sub-wavelength structures that goes beyond what can be obtained using known approaches.